The invention relates to an antenna array presenting an optimum sparse design for radio base stations in area covering communication systems.
The demand for increased capacity in area covering communication networks can be solved by the introduction of array antennas. These antennas are arrays of radiating elements that can create one or more narrow beams in the azimuth plane. A narrow beam is directed or selected towards the client of interest, which leads to a reduced interference in the network and thereby increased capacity.
A number of simultaneous fixed scanned beams may be generated in the azimuth plane by a Butler matrix connected to the antenna columns. The antenna element spacing is determined by the maximum scan angle as the creation of interference lobes due to repeated constructive adding of the phases (also referred to as grating lobes) must be considered.
A problem in designing antennas is that the radiating elements in an array antenna have to be spaced less than one wavelength apart in order not to generate grating (secondary) lobes. In the case of a scanned beam, the spacing has to be further reduced. In the limit case when the main beam is scanned to very large angles (as in the case of an adaptive antenna for mobile communications base stations), the element separation needs to be reduced to half a wavelength or less to avoid generating grating lobes within visible space.
The radiating element grid is usually either rectangular (FIG. 1) or triangular (FIG. 4). It is well known that an equilateral triangular element grid reduces the number of antenna elements with about 13% compared to a square grid assuming the same maximum scan angle without generating grating lobes. However, this element grid is not optimized for the one dimensional multi-beam scanned array case. For instance, reference to this can be found in E. D. Sharp, xe2x80x9cA triangular arrangement of planar-array elements that reduces the number neededxe2x80x9d, IEEE Trans. Antennas and Propagation, vol. AP-9, pp. 126-129, March 1961.
The radiating elements in an array antenna are often placed in a regular rectangular grid as illustrated in FIG. 1. The element spacing is denoted dx along the x-axis and dy along the y-axis. The beam directions are found by transforming from element space to beam space. The corresponding beam space for the antenna illustrated in FIG. 1 is found in FIG. 2.
In this case the main beam is pointing in the direction along the antenna normal. The beams outside the visible space (i.e. outside the unit circle) constitute grating lobes and they do not appear in visible space as long as the beam is not scanned and the element spacing is less than one wavelength along both axes (xcex/dx greater than 1 and xcex/dy greater than 1). For a large array, the number of radiating elements in the rectangular arranged grid is approximately given by NR=A/(dxdy), where A is the area of the antenna aperture.
When the main beam is scanned along the x-axis, all beams in beam space move in the positive direction by an amount, which equals a function expressed as sinus of the scan (radiating) angle. For each horizontal row in a one-dimensional scan in the x-direction we can express the secondary maxima or grating lobes as
xm=sin(xcex8s)+mxc2x7xcex/dx, m=xc2x11,xc2x12, . . .
wherein xm is the position of lobe m, xcex8s is the scan angle relative to the normal of the array and dx is the distance between the elements in the horizontal plane. As the distance between lobes here is xcex/dx it will be realized that the largest element distance for a scan angle producing no grating lobes within the visible region is       d    λ     less than       1          1      +              sin        ⁡                  (                      θ                          m              ⁢                              xe2x80x83                            ⁢              a              ⁢                              xe2x80x83                            ⁢              x                                )                    
In a case illustrated in FIG. 3, a second beam (grating lobe) enters visible space in addition to the main beam. This may be avoided by reducing the element spacing along the x-axis. When the element spacing is less than half a wavelength (i.e. xcex/dx greater than 2), no grating lobe will enter visible space independent of scan angle, since |sin(xcex8)|xe2x89xa61.
Radiating elements placed in an equilateral triangular grid are shown in FIG. 4. The vertical element spacing is defined as dy. A corresponding beam space is illustrated in FIG. 5. The element spacing must not be greater than 1/{square root over (3)} wavelengths (i.e. a maximum value of dy is about 0.58 wavelengths) along the y-axis (and 2dx is one wavelength along the x-axis [equal to dy{square root over (3)}=0.58xc2x7xcex{square root over (3)}=xcex]) to avoid generating grating lobes for any scan angle. Thus the optimum element spacing, dy, in an equilateral triangular grid of radiating elements is 1/{square root over (3)} wavelengths. For a large array, the number of radiating elements in the triangular arranged grid is approximately given by NT=A/(2dxdy). (Also see reference E. D. Sharp mentioned above.) A reduction of (NRxe2x88x92NT)/NR=13.4% is obtainable for the equilateral triangular grid compared to the square grid assuming the same grating lobe free scan volume. (NT=4A/xcex2 and NR=2A{square root over (3)}/xcex2.)
However there is still a demand for an optimization of the radiating grid in an array antenna for obtaining a sparse array antenna for communication base station antennas particularly without generating grating lobes in visible space.
The present invention discloses an antenna array for a base station for communication systems presenting a sparse element grid for one-dimensional scanning of beams or multi-beam patterns, the radiating elements partially filling a predetermined aperture of the antenna for coverage of a sector with a horizontal extension. The element spacing is governed by scanning in the x-direction mainly. In a triangular element grid the element spacing along the y-axis is increased to an order of one wavelength (dy≈xcex) still maintaining a desired aperture with low grating lobe interaction, and maintaining half a wavelength spacing along the x-axis (dx≈xcex/2). This corresponds to a reduction of radiating element by the order 50% compared to the square grid of radiating elements arranged with half a wavelength spacing. By taking into account and limiting the horizontal scan the vertical spacing may be further increased to obtain an optimum sparse antenna element grid in a created one-dimensional scanned array or a multi-beam array e.g., for communication system base stations.
Furthermore the present invention may utilize electronic down-tilting of the scanned lobes to minimize interference with nearby cells in a communication network when the sparse array antenna according to the present invention is utilized for base station operations.
A one-dimensional scanned or multi-beam antenna device according to the present invention is set forth by the attached independent claims 1, 19 and 20 and further embodiments according to claim 1 are defined by the dependent claims 2 to 18.